Semiconductor memory is widely used in various electronic devices such as cellular telephones, digital cameras, personal digital assistants, medical electronics, mobile computing devices, and non-mobile computing devices. Semiconductor memory may comprise non-volatile memory or volatile memory. A non-volatile memory allows information to be stored and retained even when the non-volatile memory is not connected to a source of power (e.g., a battery). Examples of non-volatile memory include flash memory (e.g., NAND-type and NOR-type flash memory) and Electrically Erasable Programmable Read-Only Memory (EEPROM).
A charge-trapping material can be used in non-volatile memory devices to store a charge which can correlate to a data state. The charge-trapping material can be arranged vertically in a three-dimensional (3D) stacked memory structure. One example of a 3D memory structure is the Bit Cost Scalable (BiCS) architecture which comprises a stack of alternating conductive and dielectric layers. A memory hole is formed in the stack and a NAND string is then formed by filling the memory hole with materials including a charge-trapping layer to create a vertical column of memory cells. Control gates of the memory cells are provided by the conductive layers. The threshold voltage (Vt) of the memory is controlled by the amount of charge that is retained in the charge-trapping material. That is, the minimum amount of voltage that must be applied to the control gate before the memory cell is turned on to permit conduction between its source and drain is controlled by the level of charge retained in the charge-trapping material. As such, the threshold voltage is used to an indicator of the data being stored by a memory cell.
Some non-volatile memory devices are used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two ranges of threshold voltages that correspond to two data states: an erased state (e.g., data “1”) and a programmed state (e.g., data “0”). Such a device is referred to as a binary or two-state device.
A multi-state (or multi-level) non-volatile memory is implemented by identifying multiple, distinct allowed ranges of threshold voltages. Each distinct range of threshold voltages corresponds to a data state assigned a predetermined value for the set of data bits. The specific relationship between the data programmed into the memory cell and the ranges of threshold voltages depends upon the data encoding scheme adopted for the memory cells. For example, U.S. Pat. No. 6,222,762 and U.S. Patent Application Publication No. 2004/0255090 both describe various data encoding schemes for multi-state flash memory cells. While multi-state non-volatile memory can store more data than binary non-volatile memory, the process for programming and verifying the programming can take longer for multi-state non-volatile memory.